Why is ice slippery?

A thin layer of liquid water is what makes ice slippery and not directly the ice itself. Liquid water naturally forms on the surface of all ice, even at very low temperatures, because of the nature of surface chemical bonds. Public Domain Image, source: Christopher S. Baird.

On its own, ice is not slippery. When you step onto an icy sidewalk, you do indeed feel a slippery surface. But the slipperiness is caused by a thin layer of liquid water and not directly by the solid ice itself. Water on a smooth surface is slippery because water is a low-viscosity liquid. As such, there are no permanent intermolecular bonds in liquid water, and the transient intermolecular bonds are weak. This means that water molecules can move about freely, slide past each other easily, and fill any microscopic holes or cracks that would snag an object.

But how does water get on the ice in the first place? This is a harder question to answer. For many years, scientists thought that the cause was pressure. When you apply pressure to water, you do indeed lower its melting point. In 1886, engineer John Joly proposed that the weight of a person skating on ice created enough pressure to lower the melting point of ice below the ambient temperature, thus causing the ice to melt to a thin layer of water under the skates. But this explanation does not hold up to closer scrutiny. No matter how much pressure you apply, you can't lower the melting point of water below -22 degrees Celsius. And yet, ice at temperatures lower than this still exhibits the formation of a slippery liquid layer. While pressure can play a role, it is not the dominant effect.

In 1939, Frank P. Bowden and T. P. Hughes proposed that the friction of objects sliding on the ice causes it to heat up enough to melt. While friction does indeed cause heat, it does not create enough heat to melt ice that is at a very low temperature. And yet, ice at low temperature still forms a slippery liquid layer. Also, frictional heating cannot explain why a slippery layer forms even if the object is perfectly motionless. While frictional heating can play a role in reducing friction for objects at high speed, it is not the dominant effect.

The exact details of why ice forms a slippery liquid layer, even when little pressure or friction is applied, is currently not completely settled by scientists and is an ongoing area of research. Ice has been found to always have a thin liquid layer on its surface, even at temperature well below freezing, and even in the absence of contact with any objects. The formation of a surface liquid layer is therefore a property of ice itself and not a property of interacting with an object. The liquid layer seems to arise because the water molecules at the surface have fewer chemical bonds than in bulk, so they can better absorb natural vibrations. In a 2005 Physics Today review article, Robert Rosenberg states, "The periodic structure breaks down [at the surface] and the molecular layers adopt a more amorphous reconstruction in response to the reduced number of chemical bonds holding the surface molecules in place. Atoms in the outermost surface vibrate with a greater amplitude as a function of temperature than atoms in the interior lattice... Surface melting is attributable to the interaction of the vibrational motion of the surface molecules with the interior bulk molecules."